1. Field of the Invention
The present invention relates to a semiconductor device and more particularly to a semiconductor device having a short circuit protecting function for preventing a short circuit of a semiconductor element such as an insulated gate bipolar transistor (IGBT).
2. Description of the Background Art
In an IGBT in which a large current flows as a main current, a sense emitter to be used for measuring a current is provided separately from an emitter in which the main current flows, and the main current can be indirectly monitored by measuring a current (sense current) of the sense emitter with a conversion into a voltage through a current and voltage converting section.
As an example of the current and voltage conversion, FIG. 9 shows a structure using a resistor. In FIG. 9, an IGBT 1 having a collector C connected to a terminal T1 and an emitter E connected to a terminal T2 is provided, and has a sense emitter SE connected to the terminal T2 through a resistor R101 to be a current and voltage converting section CV1. A terminal T5 for outputting a sense emitter voltage is connected to an end on the sense emitter SE side of the resistor R101. The IGBT 1 has a gate connected to a terminal T3.
As an example of the current and voltage conversion, FIG. 10 shows a structure using a current mirror circuit. In FIG. 10, the same structures as those in FIG. 9 have the same reference numerals and repetitive description will be omitted.
In FIG. 10, a sense emitter SE of an IGBT 1 is connected to a collector of a transistor Q2 constituting a current and voltage converting section CV2. The transistor Q2 is paired with a transistor Q1 to constitute the current mirror circuit, and both gates are connected to the sense emitter SE in common and both emitters are connected to a terminal T2 in common.
A collector of the transistor Q1 is connected to a positive electrode of a constant voltage source PS through a resistor R102, a negative electrode of the constant voltage source PS is connected to the terminal T2, and a terminal T5 for outputting a sense emitter voltage is connected to an end on the transistor Q1 side of the resistor R102.
The current and voltage converting section CV2 constituted by the current mirror circuit can lessen a voltage drop with a current and voltage conversion to carry out a measurement.
FIG. 11 is a diagram showing a structure of a short circuit protecting circuit for measuring a sense emitter voltage to protect a short circuit by using a resistor R101 as a current and voltage converting section.
In FIG. 11, a sense emitter voltage output from an end on the sense emitter SE side of the resistor R101 is input to an input terminal on the positive side of a comparator C101 and a positive electrode of a constant voltage source PS is connected to an input terminal on the negative side of the comparator C101.
An output terminal of the comparator C101 is connected to a cathode of a diode D101 and an anode of the diode C101 is connected to a gate of an IGBT 1.
In a short circuit protecting circuit having such a structure, a predetermined voltage supplied from the constant voltage source PS is compared with the sense emitter voltage. If the sense emitter voltage is higher than the predetermined voltage, it is decided that a sense emitter current abnormally flows. Consequently, a current is caused to flow from the gate of the IGBT 1 toward an emitter through the diode D101, thereby dropping a gate voltage and reducing a collector current of the IGBT 1.
There is a problem in that the sense emitter current is assumed to have a uniquely simple correlation with the collector current.
More specifically, an electric potential of the sense emitter SE is increased with the detection of the sense current in an actual circuit. For example, therefore, in some cases in which an electric potential of a collector C is low, the electric potentials of the collector C and the sense emitter SE become insufficient so that the sense emitter current is measured to be small even if a large collector current flows. For this reason, the detection precision of the collector current is reduced and a sufficient protection cannot be obtained in some cases. In addition, a rise in a collector voltage rapidly increases the sense emitter current with a start of a protecting operation so that the protecting operation is promoted. Furthermore, a positive feedback in which the gate voltage is decreased to increase the collector voltage is generated and the sense emitter current is rapidly increased with a delay of the operation of a protection circuit, thereby causing a malfunction in some cases.
Moreover, the collector current is rapidly reduced by the inaccurate detection of the collector current. In the IGBT to be used for controlling a large current, for example, there is a problem in that a high surge voltage is generated.
In order to solve these problems, a structure for detecting a collector voltage has also been proposed. However, the collector of the IGBT is provided on the high potential side. Therefore, a detection circuit corresponding to a high voltage is required so that a manufacturing cost is increased and a large space for providing the detection circuit is required, resulting in an increase in a size of the device. Moreover, a countermeasure to be taken against a noise made from various circuits on the high potential side becomes complicated.
Japanese Patent Application Laid-Open No. 11-299218 (1999) has disclosed a structure in which a sense emitter and an emitter in an IGBT are imaginarily short-circuited by using an operational amplifier, thereby preventing a fluctuation in a sense emitter voltage. In order to implement such a structure, it is necessary to prepare a power supply having a lower potential than an emitter potential. In addition, in the case in which the structure is to be implemented as a monolithic IC, there is a problem in that a countermeasure to be taken against a noise becomes complicated because an emitter potential to be an original grounding potential is an intermediate potential of a power supply of the monolithic IC.
A first aspect of the present invention is directed to a semiconductor device comprising a transistor having a sense emitter, and a protecting system for carrying out a protecting operation for detecting a sense current flowing to the sense emitter to control a collector current, the protecting system including a current and voltage converting section having a variable resistor for converting the sense current into different voltages by change of a resistance value thereof, thereby generating a plurality of sense voltages, and a current ratio detecting section for receiving the sense voltages output from the current and voltage converting section and information about a resistance value of the variable resistor corresponding to the sense voltages, thereby calculating a ratio of each of the sense voltages to each of the resistance values and detecting an amount of change of the ratio thus calculated.
A second aspect of the present invention is directed to the semiconductor device according to the first aspect of the present invention, wherein a resistance value of the variable resistor is varied in response to a voltage signal which is changed on a time basis, the information about the resistance value of the variable resistor is the voltage signal which is changed on a time basis, and the resistance value of the variable resistor and the voltage signal which is changed on a time basis have a proportional relationship.
A third aspect of the present invention is directed to a semiconductor device comprising a transistor having a sense emitter, and a protecting system for carrying out a protecting operation for detecting a sense current flowing to the sense emitter to control a collector current, the protecting system including a current and voltage converting section for converting the sense current into a voltage and outputting the voltage, a forcible voltage changing section for applying external voltages having different values to the sense emitter and forcibly changing a sense emitter potential by the current and voltage converting section, thereby outputting the voltages thus changed as a plurality of sense voltages from the current and voltage converting section, and a current ratio detecting section for receiving the sense voltages output from the current and voltage converting section and information about the external voltages corresponding to the sense voltages, thereby calculating a ratio of each of the sense voltages to each of the external voltages and detecting an amount of change of the ratio thus calculated.
A fourth aspect of the present invention is directed to the semiconductor device according to the third aspect of the present invention, wherein the current and voltage converting section is a current mirror circuit having an input section connected to the sense emitter, the external voltage is generated by a variable resistor connected between an output end of a voltage source of the current mirror circuit and the sense emitter, a resistance value of the variable resistor is changed in response to a voltage signal which is changed on a time basis, the information about the external voltage is the voltage signal which is changed on a time basis, and the resistance value of the variable resistor and the voltage signal which is changed on a time basis have a proportional relationship.
A fifth aspect of the present invention is directed to a semiconductor device comprising a transistor having a sense emitter, and a protecting system for carrying out a protecting operation for detecting a sense current flowing to the sense emitter to control a collector current, the protecting system including a current and voltage converting section having a resistance variable section for converting the sense current into different voltages by change of a resistance value thereof, thereby generating a plurality of sense voltages, a voltage comparator serving to input the sense voltages output from the current and voltage converting section and having two high and low comparison potentials set by a hysteresis, and connecting means for electrically connecting a gate of the transistor to a predetermined electric potential based on an output of the voltage comparator.
A sixth aspect of the present invention is directed to the semiconductor device according to the firth aspect of the present invention, wherein the resistance variable section includes first and second resistors connected in series between the sense emitter and an emitter of the transistor, and a switch element electrically connected in parallel with one of the first and second resistors, an ON/OFF operation of the switch element being controlled based on the output of the voltage comparator.
A seventh aspect of the present invention is directed to the semiconductor device according to the fifth aspect of the present invention, wherein the connecting means is a diode having a cathode connected to the output of the voltage comparator and an anode connected to the gate of the transistor.
An eighth aspect of the present invention is directed to a semiconductor device comprising a transistor having a sense emitter, and a protecting system for carrying out a protecting operation for detecting a sense current lowing to the sense emitter to control a collector current, the protecting system including a current and voltage converting section having a resistance variable section for converting the sense current into different voltages by change of a resistance value thereof, thereby generating a plurality of sense voltages, first and second voltage comparators for inputting the sense voltages output from the current and voltage converting section, an extension circuit connected to an output side of the first voltage comparator and serving to extend a time required for change such that a time in which an output of the first voltage comparator is changed from a second potential to a first potential is longer than a time in which the output of the first voltage comparator is changed from the first potential to the second potential, a variable d.c. power supply for changing a comparison voltage of the second voltage comparator based on the output of the first voltage comparator, and connecting means for electrically connecting a gate of the transistor to a predetermined electric potential based on an output of the second voltage comparator.
A ninth aspect of the present invention is directed to the semiconductor device according to the eighth aspect of the present invention, wherein the resistance variable section includes first and second resistors connected in series between the sense emitter and an emitter of the transistor, and a switch element electrically connected in parallel with one of the first and second resistors, an ON/OFF operation of the switch element being controlled based on the output of the first voltage comparator.
A tenth aspect of the present invention is directed to the semiconductor device according to the eighth aspect of the present invention, wherein the connecting means is a diode having a cathode connected to the output of the second voltage comparator and an anode connected to the gate of the transistor.
An eleventh aspect of the present invention is directed to a semiconductor device comprising a transistor having at least two sense emitters, and a protecting system for carrying out a protecting operation for detecting a sense current flowing to the at least two sense emitters to control a collector current, the protecting system including a current and voltage converting section for converting the sense current flowing to the at least two sense emitters into voltages, thereby generating at least two sense voltages which are different from each other, and a current ratio detecting section for receiving the at least two sense voltages output from the current and voltage converting section, thereby calculating a ratio of the at least two sense voltages.
A twelfth aspect of the present invention is directed to the semiconductor device according to the eleventh aspect of the present invention, wherein the current and voltage converting section includes at least two resistors having different resistance values which are connected between the at least two sense emitters and an emitter of the transistor, respectively.
According to the first aspect of the present invention, the current ratio detecting section receives a plurality of sense voltages output from the current and voltage converting section and information about the resistance value of the variable resistor corresponding to the sense voltages, thereby calculating the ratio of each of the sense voltages to each of the resistance values and detecting the amount of change thereof. Thus, a pattern of a short circuit state of the transistor can be known based on the amount of change. As a result, a short circuit protecting operation is carried out corresponding to the pattern of the short circuit state. Thus, it is possible to implement the short circuit protecting operation in consideration of a difference in the short circuit state depending on a magnitude of a collector voltage. Moreover, it is not necessary to detect the collector voltage. Therefore, a detection circuit corresponding to a high voltage and a large space for providing the detection circuit are not required, and a manufacturing cost can be reduced and a size of the device can be prevented from being increased.
According to the second aspect of the present invention, it is possible to obtain a practical structure for changing the resistance value of the variable resistor. Moreover, the information about the resistance value of the variable resistor is given through the voltage signal which is changed on a time basis and has a proportional relationship with the resistance value of the variable resistor. Therefore, it is possible to easily carry out an operational processing or the like in the current ratio detecting section.
According to the third aspect of the present invention, a plurality of sense voltages output from the current and voltage converting section and information about external voltages corresponding to the sense voltages are received to calculate a ratio of each of the sense voltages to each of the external voltages and to detect the amount of change thereof. Consequently, it is possible to know a pattern of a short circuit state of the transistor based on the amount of change. As a result, a short circuit protecting operation is carried out corresponding to the pattern of the short circuit state. Thus, it is possible to implement the short circuit protecting operation in consideration of a difference in the short circuit state depending on a magnitude of a collector voltage. Moreover, it is not necessary to detect the collector voltage. Therefore, a detection circuit corresponding to a high voltage and a large space for providing the detection circuit are not required, and a manufacturing cost can be reduced and a size of the device can be prevented from being increased.
According to the fourth aspect of the present invention, the current mirror circuit is used for the current and voltage converting section. Consequently, it is possible to lessen a drop in a voltage with a current and voltage conversion, thereby measuring an emitter voltage. Moreover, a voltage source of the current mirror circuit is used for generating an external voltage and a value thereof is changed through the variable resistor. Therefore, it is possible to simply obtain a structure required for the application of the external voltage and to prevent a manufacturing cost from being increased.
According to the fifth aspect of the present invention, the voltage comparator compares each of the sense voltages output from the current and voltage converting section with two high and low comparison potentials set by a hysteresis. Consequently, in the case in which the sense voltage for each resistance value exceeds one of the comparison potentials but does not exceed the other comparison potential or exceeds both of the two comparison potentials, a sense voltage characteristic can be analyzed in more detail. Therefore, a short circuit protecting operation can be carried out more properly. Moreover, the gate of the transistor is electrically connected to the predetermined electric potential through the connecting means based on the output of the voltage comparator. In the case in which the short circuit protecting operation is required, consequently, the gate of the transistor is connected to the predetermined electric potential to decrease a gate current. Thus, a collector current can be reduced to carry out the short circuit protection.
According to the sixth aspect of the present invention, it is possible to obtain a practical and simple structure of the resistance variable section.
According to the seventh aspect of the present invention, it is possible to obtain a practical and simple structure of the connecting means.
According to the eighth aspect of the present invention, there are provided the first and second voltage comparators to which the sense voltages output from the current and voltage converting section are input. Based on the output of the first voltage comparator, the comparison voltage of the second voltage comparator is changed through the variable d.c. power supply. Therefore, the sense voltage can be compared with at least two comparison voltages. Thus, it is possible to carry out the short circuit protecting operation more properly. Moreover, the extension circuit is provided. Therefore, it is possible to extend the period for detection of the second comparator to determine the execution and stop of the protecting operation. Thus, it is possible to prevent a malfunction from being caused by a short period for detection.
According to the ninth aspect of the present invention, it is possible to obtain a practical and simple structure of the resistance variable section.
According to the tenth aspect of the present invention, it is possible to obtain a practical and simple structure of the connecting means.
According to the eleventh aspect of the present invention, the current ratio detecting section calculates the ratio of at least two sense voltages output from the current and voltage converting section. Therefore, the sense voltages are directly compared with each other. Consequently, a time required for knowing the pattern of the short circuit state can be reduced and the protecting operation can be carried out in a real time.
According to the twelfth aspect of the present invention, it is possible to obtain a practical and simple structure of the current and voltage converting section.
In order to solve the above-mentioned problems, it is an object of the present invention to provide a semiconductor device comprising a short circuit protecting system capable of enhancing the detection precision of a collector current, thereby carrying out a reliable short circuit protection.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.